1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method thereof, and a computer program.
2. Description of the Related Art
Digital processing of medical radiation fluoroscopic images has become common due to recent advancements in digital technology. Especially, conventional radiation diagnosis, which uses film for radiation photographing, has replaced with newly developed two-dimensional radiation sensor technology that allows outputting radiation images as digital data. When such sensors are used in radiation fluoroscopy apparatus, digital imaging processes such as tone processing for improving visibility of radiation images has become an essential technology.
Also, Auto Exposure Control (AEC) is conducted in radiation fluoroscopy, which used for detecting X-ray dosage transmitted by an object and controlling X-ray dosage without an excess or deficiency.
In both image processing and exposure controlling as mentioned above, the degree of processing conducted on the region of interest regarding the diagnosis, is decided based on values that represent the feature level (feature amounts). As an example, in tone processing, maximum density value and minimum density value of the pixel within the region of interest of the radiation image is calculated, then the maximum density value and minimum density value is controlled so that brightness of the monitor output is within the upper and lower bounds. Transforming the image based on the output as above is called output transformation.
In addition, Auto Exposure Control (AEC) extracts feature amounts such as average value from the region of interest of radiation fluoroscopic image created by radiation exposure by the radiation generator. Then, the next X-ray exposure condition (tube voltage, tube current, pulse width, etc. of the radiation generator) is controlled based on comparison of the feature amount with a reference value.
It is necessary to select the area of interest used for compute feature amount of the radiation image to conduct adequate image processing and auto exposure control.
However, in general radiation fluoroscopy, exposed X-ray dosage is controlled by exposure field aperture attached to radiation generator by controlling radiation exposure-field region for reducing the influence of radiation decrease and scattered radiation. When such controlling is conducted, the region on the radiation image, outside of opening of the exposure field aperture (hereinafter: outer exposure field region) does not contain effective object information. Because this region does not contain information obtain from radiation directly pass through the object; it only contain the information obtained from scattered radiations entered the sensor.
In addition, when radiopaque objects such as artificial tooth root (dental implantations), and heart phase maker exist in the photographed human body, image density value differs significantly on the radiation image from the interested photographed body area.
When the region of interest for feature calculation includes outer exposure field regions and regions corresponding to radiopaue objects, feature amounts that should show the features of object region with diagnostic interest get influenced by the regions that do not contain valid information in regards to the diagnostic. This become a hindrance for appropriate image processing and auto exposure control (AEC). To overcome this problem, methods (for exposure field recognition process, object recognition process) that can clear off outer exposure field region and radiopaque object region from the region of interest and then recognize the required region, have been proposed.
As an example, in Japanese Patent Laid-Open No. 63-259538, proposes a recognition process of exposure field by utilizing steep change of image density value between the border (outline of exposure field) of exposure field region and outer exposure field region. In this method, the image is scanned from a predetermined point (as a example the center point of the image) assumed to be within exposure-field region of the image, on a straight line while advancing towards the end of the image. When the difference value of a pixel scanned on the straight line is over a predetermined threshold value, the element is set as the slate point on an outline of exposure field. Then, slate points marked by neighboring lines are connected and considered as the outline of exposure field, and inside of the outline is defined as exposure field region and outside of the outline is defined as the outer exposure field region.
Moreover, in Japanese Patent Laid-Open No. 10-275213, instead of selecting slate points on outline of the exposure field using the method by Japanese Patent Laid-Open No. 63-259538, straight line scanning based on Hough transform is used to increase the detection precision of exposure field region.
In addition, in Japanese Patent Laid-Open No. 2006-254934 proposed a method, to prevent the error may occurred in X-ray exposure unit while extracting the border in method described in Japanese Patent Laid-Open Nos. 10-275213 and 63-259538, first the image is partitioned using the straight lines which creates the slate points on outline of exposure field and then the image-intensity is evaluated in partitioned image parts to detect the exposure field region.
As a method that clears off the radiopaque object region, Japanese Patent Laid-Open No. 2006-254934 is proposed an object recognition method using region growing algorithm. In this method, the initial region is selected by analyzing the object region, after deleting outer exposure field region and direct irradiation region that directly arrived to the sensor without passing the object. From this initial region, based on difference value of adjacent pixels initial region expanded while detecting radiopaque object region, and manage to separate object region and radiopaque object as a result.
In the methods discussed above various parameters are needed for calculations. Such as threshold value needed decide outline of exposure field, threshold value necessary for detecting straight lines in Hough transformation, parameters necessary for deciding likeness of exposure field region or image partitioning and growing conditions which differ according to the image processing state can be given as some of these parameters.
In general, image-processing methods that separate the region necessary and the region unnecessary are constructed with multiple algorithms to increase the detection precision. For this reason, the processing time increase is considered as a problem. Moreover, processes require large number of parameters, and tuning these parameters accordingly is also considered as a problem.
Thus, in present invention, an exposure field region and object region recognition technique with short processing time and easy parameter tunings is proposed.